The present invention relates to a method of checking the beam generation means and beam acceleration means of an ion beam therapy system that is operated especially with heavy ions.
Ion beam therapy systems are preferably used in the treatment of tumours. An advantage of such systems is that, on irradiation of a target object (target), the major portion of the energy of the ion beam is transferred to the target, while only a small amount of energy is transferred to healthy tissue. A relatively high dose of radiation can therefore be used to treat a patient. X-rays, on the other hand, transfer their energy equally to the target and to healthy tissue, so that for health reasons, for the protection of the patient, it is not possible to use high doses of radiation.
There is known from U.S. Pat. No, 4,870,287, for example, an ion beam therapy system in which there are generated from a proton source proton beams of which the protons can be delivered to various treatment or irradiation sites by an acceleration device. Provided at each treatment site is a rotating cradle having a patient couch so that the patient can be irradiated with the proton beam at different angles of irradiation. While the patient is spatially located in a fixed position inside the rotating cradle, the rotating cradle revolves round the body of the patient in order to focus the treatment beams at various angles of irradiation onto the target located in the isocentre of the rotating cradle. The acceleration device comprises a combination of a linear accelerator (LINAC) and a so-called synchrotron ring.
In H.F. Weehuizen et al. CLOSED LOOP CONTROL OF A CYCLOTRON BEAM FOR PROTON THERAPY, KEK Proceedings 97-17, January 1998, a method of stabilising the proton beam in proton beam therapy systems is proposed in which the treatment beam is actively so controlled that it is located on the centre line of the corresponding beam delivery system at two measurement points spaced from each other in the longitudinal direction. The first measurement point is located between a pair of deflection magnets and is formed by a multi-wire ionisation chamber. Depending on the actual value of the beam position delivered from that multi-wire ionisation chamber relative to the centre point of the beam path, a PI control is generated by further deflection magnets arranged upstream from the first-mentioned pair of deflection magnets. The second measurement point is located just upstream of the isocentre and is formed by an ionisation chamber which is divided into four quadrants. Depending on the actual position value of that ionisation chamber, again PI control signals are generated, but those control signals are intended for the first-mentioned deflection magnets. Such a control arrangement is said to render possible both angle stability in terms of the centre line of the beam delivery system and lateral position stability of the proton beam.
When, however, heavy ion irradiation is carried out, that is to say irradiation with ions that are heavier than protons, large and heavy devices are necessary, with the result that there is a tendency to avoid the use of rotating cradles and instead move the patient or the patient couch. Corresponding therapy systems are described, for example, in E. Pedroni: Beam Delivery, Proc. 1st Int. Symposium on Hadrontherapy, Como, Italy, Oct. 18-21, 1993, page 434. Such systems are accordingly eccentric systems.
Since, however, mainly isocentric systems are preferred by oncologists, a heavy ion beam therapy system was proposed in which, although rotating cradles are used at the treatment sites, the radii of the rotating cradles can be reduced by virtue of the treatment beam delivered to each rotating cradle horizontally along its axis of rotation being so guided by means of suitable magnet and optics arrangements that, for the irradiation of a target, the beam is first of all directed away from the axis of rotation and later crosses the axis of rotation again in the isocentre. There is provided for the irradiation of the target a grid scanner, which comprises vertical deflection means and horizontal deflection means, each of which deflects the treatment beams at right angles to the beam axis, with the result that an area surrounding the target is scanned by the treatment beams. Such a system thus essentially provides beam guidance in only one plane of the rotating cradle.
The irradiation by the grid scanner is carried out with the aid of radiation dose data that are calculated automatically by the supervisory control system of the ion beam therapy system according to the patient to be irradiated or treated.
Since a high level of operational safety and operational stability in terms of the treatment beam is necessary in ion beam therapy systems, a monitoring device for monitoring the treatment beam delivered by the grid scanner is provided in the afore-described heavy ion beam therapy system. The monitoring device is arranged between the last deflection magnets of the above-mentioned magnet arrangement and the isocentre, and can comprise ionisation chambers for monitoring the particle flow and multi-wire chambers for monitoring the beam position and the beam width.
For safety reasons, various DIN standards have to be observed in the operation of medical electron accelerators. Those standards are concerned on the one hand with the inspection test, that is, the inspection of the readiness for operation, and on the other hand with the consistency test, that is, examination of operational stability, of the system. For ion beam therapy systems, especially for heavy ion beam therapy systems, safety standards of that kind developed specifically for such systems are not yet known, but there is still a need, in ion beam therapy systems too, for as high as possible a level of operational safety and operational stability.
The problem underlying the present invention is therefore to propose a method of checking the beam generation means and the beam acceleration means of an ion beam therapy system in order to improve operational safety and operational stability, especially in respect of the beam generation means and the beam acceleration means. The process shall at the same time be suitable especially for use with heavy ions.
The problem is solved in accordance with the present invention by a method having the features of Claim 1. The dependent claims each define preferred and advantageous embodiments of the present invention.
According to the present invention, an ion beam therapy system that has at least one ion source, one acceleration device and one beam guidance system is operated, wherein the type of ion, the ion beam energy, the ion beam intensity and the blocking of the accelerator and also the means for terminating extraction are checked. For that purpose, the type of ion is checked by recording the charge spectrum of the ion source in a high-charge injector and comparing it with a reference spectrum. The ion beam energy is checked by measuring Bragg curves in selected energy levels in an absorber ionisation chamber system. The ion beam intensity is checked by measuring the particle intensity in the accelerator device in an upper range of intensity, and measuring the particle intensity at the irradiation site for all planned levels of intensity. The blocking of the accelerator is checked by sending switch-off commands and adjustment commands to components that are deactivated beforehand.
The means for terminating extraction are checked by generating an interlock condition during an ion extraction and measuring decay time of the ion extraction.
In the present invention there are used especially 12C2+ ions, which are stripped to 12C6+ in the beam guide between a linear accelerator and a synchrotron ring. For that purpose, a stripper is provided downstream of the linear accelerator. Owing to their physical and biological properties, those carbon ions have proved to be very effective in the treatment of tumours and have the advantages of a high physical selectivity and a high biological effectiveness and, in addition, offer the possibility of verification of the irradiation with the aid of a positron emitter tomograph (PET). By suitable selection of the carbon ions, the biological effectiveness can be controlled in such a manner that it is low in the plateau region of the Bragg""s curve and high in the region of the Bragg peak. Consequently, the target or the tumour can be treated with a comparatively high dose while the dose for the surrounding healthy tissue is minimised.
In order to ensure the use and acceleration exclusively of the type of ion intended, the charge spectrum of the beam present is recorded and evaluated in the high-charge injection system. By comparison of the recorded charge spectrum with a reference spectrum, undesired ions or irregularities can be detected and appropriate measures taken. This check can be carried out, for example, with each initialisation of an ion source.
The linear accelerator is used for the initial acceleration of the ions fed to it, those ions then being delivered by an injection line to a synchrotron. The injection line comprises, in addition to the stripper already mentioned, a further pulse generator arrangement for precise shaping of the injection pulses, magnetic dipoles for charge analysis, quadrupoles for adapting the radiation to the receiving capacity of the synchrotron etc.
The synchrotron ring serves for the final acceleration of the ions fed to it to a determined energy and comprises, for example, a plurality of deflection magnets, quadrupoles and sextupoles. Arranged inside the synchrotron is a cooling means. By means of repeated injection cycles, the injected ions are accelerated from an energy in the region of a few MeV/u to an energy of, for example, more than 400 MeV/u. The treatment beam accelerated in that manner is extracted at a particular point in the synchrotron by way of a high energy beam guidance channel and delivered to the individual treatment sites.
There has been developed for the above-described ion beam therapy system an extensive checking system, to be described in detail hereinbelow, for checking and controlling the important performance features of the therapy system.
In addition to the inspection, already described hereinbefore, of the type of ion, at the same time the radiation energy of the treatment beam is monitored. This is a requirement since it is necessary to adhere to the radiation energies required by the particular therapy. For that purpose, the monitoring means indicated comprises an absorber ionisation chamber system allocated to the isocentre of the respective treatment site. The absorber ionisation chamber system measures the position of the Bragg peak at the treatment site for a few selected energy levels, which are activated during a therapy test cycle, the instantaneous radiation energy being derived from the measured position of the Bragg peak. In order to determine the position of the Bragg peak, the Bragg curves are measured in precise steps. If, on examination, there were to be a departure of the Bragg peak from the desired position of more than 0.5 mm, then intervention would be necessary. In order to examine consistency, the described checking procedure can be carried out prior to each block of irradiation procedures.
A further point of detail with regard to inspection of the treatment beam concerns the monitoring of the level of intensity of the slowly extracted treatment beam at the irradiation or treatment site. The limited dynamics of the grid scanner puts an upper limit on the scanning speed of the scanned treatment beam, the component that determines that limitation being the maximum current-increase speed of the magnet current supply devices. The scanning speed of the treatment beam depends on the particular intensity of the beam and the planned dose of particles. In order to ensure that the maximum scanning speed is not achieved during the irradiation, the particle rate extracted from the synchrotron 5 is not permitted substantially to exceed the desired value. If, on the other hand, the rate falls distinctly short of that value, the total irradiation time is extended, the supervisory control and surveillance or monitoring system in that case optionally being operated in the range of very small input currents, which can adversely affect the accuracy of the beam detection. Accordingly, in the present therapy system, measurement and protocolling of the particle intensities in the synchrotron is provided in the upper intensity range and measurement and recording of the particle rate delivered to the irradiation site is provided for all levels of intensity for a plurality of energies over a few minutes. The particle rate fed from the accelerator to the irradiation site is between 2xc3x97106 and 2xc3x97108 ions per extraction from the synchrotron 5. The departure of the particle rate from the predetermined desired value may be a maximum of 30% above and a maximum of 50% below that value. If those limit values are transgressed, an appropriate intervention is necessary. In order to check the consistency of the therapy system, such an inspection may, for example, be carried out daily.
The same dependencies of energy variation, intensity variation and focusing variation must be taken as a basis for data supply for the accelerator, for irradiation planning and for grid scan programming. In order to ensure that that is the case, the data inputs generated accelerator-wise after the last therapy programming should be compared with those used for the grid scan programming and irradiation planning. Departure from those data inputs is not permissible. In order to check consistency, such a check should be carried out prior to each block of irradiation procedures.
During irradiation, the sections of the accelerator that are necessary for the therapy are blocked against (external) interference in order to avoid intentional and unintentional false settings. At the same time, operational states are activated for all components and desired value data for the apparatus deposited in the memories, e.g. EPROMS, exclusively, are accessed. The function of blocking the accelerator when interference is present can be checked by setting up a xe2x80x9csuper cyclexe2x80x9d that contains both test and therapy accelerators. Monitoring means or detectors, such as, for example, (described in detail hereinafter) profile grids, luminous targets and ionisation chambers, are moved into the high energy beam guide for the rotating cradle, and beam-influencing elements of the high energy beam guidance channel and of the synchrotron for the therapy accelerator are deactivated. Blocking of the accelerator is then activated and all test accelerators are deactivated, while the therapy accelerator is activated. In addition, all previously deactivated components are activated for the therapy accelerator, and the inserted profile grids, luminous targets and ionisation chambers are moved out again. Subsequently, switch-off commands are sent to individual magnets and adjustment commands are sent to beam guidance diagnosis components, those commands normally not being allowed to have any effect owing to the blocking of the accelerator. There is otherwise an error, which must be corrected accordingly. This check can be carried out prior to each block of irradiation procedures in order to check consistency.
It must be possible, for safety reasons, for the extraction of the treatment beam from the synchrotron 5 to be terminated within less than 1 ms after an appropriate signal from an interlock unit of the therapy system. This is effected by a special quadrupole in the synchrotron rapidly being switched off. The time between a request by the supervisory control and safety system for the beam to be terminated and the absence of the beam at the irradiation site is of crucial importance both for the grid scanning operation when there is a change between successive isoenergy levels, those levels corresponding to areas to be irradiated with constant energy, and for a possible emergency shutdown of the system in case of error. There is accordingly provided a test that measures the total time, that is to say both the reaction time of the request and the reaction time of the beam termination. To that end, the supervisory control system generates an appropriate signal which simulates the ending of an isoenergy level, or an interlock condition, that is to say a condition for an emergency shutdown, is generated. The particle count after a termination is then measured by the supervisory control system, wherein 1 ms after termination the count is not permitted to be greater than 104 particles/s. In addition, using a storage oscillograph and a pulser, which are installed in fixed position in the technical supervisory control room of the therapy system, a measurement is carried out that evaluates the output signal of the current voltage converter of one of the ionisation chambers in order to check the afore-described measurement of the supervisory control system. In that second measurement, too, it should not be possible for any beam to be detected 1 ms after termination. The following time checks during a termination should be made one after another: the beginning of the extraction time, the middle of the extraction time, the end of the extraction time and beyond the extraction time. The check should be carried out daily as a consistency check.
At the end of each irradiation procedure it is necessary, in respect of the accelerator, for a protocol to be drawn up that documents both the settings of important accelerator components during the irradiation procedure and selected beam diagnosis measurement results. In order to test the functionality of the protocolling and the protocol contents, it is proposed that a reference therapy cycle be activated and that the protocol program be called up. The protocol data drawn up by the protocol program can then be compared with the expected data, intervention being necessary when the protocol is incomplete or when a protocolled apparatus error exists. In order to check consistency, this checking procedure can be carried out prior to each block of irradiation procedures.
It is proposed especially that the calculated radiation dose values be checked for a plurality of measurement points of the phantom, adequate accuracy of the calculation of the radiation dose data being inferred when the average discrepancy between the calculated and measured values of the radiation dose for all measurement points does not exceed a predetermined first tolerance value and when for each individual measurement point the discrepancy between the calculated and the measured radiation dose for that measurement point does not exceed a predetermined second tolerance value. The first tolerance value is xc2x15% and the second tolerance value xc2x17%.
In order to check for a correct transfer of the geometric structures at the treatment site and to check the planning parameters of an image-forming device of the ion beam therapy system up to the time of positioning, a digital reconstruction, especially an X-ray reconstruction, can be calculated by the phantom, which reconstruction is compared with an X-ray image generated by the phantom in order to ascertain a possible discrepancy.
The present invention renders possible a clear improvement in the operational stability and operational safety of an ion beam therapy system and defines a checking plan having particular checking aspects that can be performed in the sense of an inspection test and/or a consistency test of the ion beam therapy system. This relates especially to irradiation planning, in the course of which radiation dose data are automatically calculated in the ion beam therapy system according to the patient to be irradiated or treated.